1. Field of the Invention
The invention relates to the field of wireless communications. More particularly, the invention relates to systems and methods for transmit power control and sub-carrier allocation in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
Wireless communication systems are continually relied upon to transmit enormous amounts of data in a variety of operating conditions. The amount of frequency spectrum, or bandwidth, that is allocated to a communication system is often limited by government regulations. Thus, there is a constant need to optimize data throughput in a given communication bandwidth.
The problem of optimizing data throughput in a given communication band is compounded by the need to simultaneously support multiple users. The users may each have different communication needs. One user may be transmitting low rate signals, such as voice signals, while another user may be transmitting high rate data signals, such as video. A communication system can implement a particular method of efficiently utilizing a communication band to support multiple users.
Wireless communication systems can be implemented in many different ways. For example, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiplexing (OFDM) are used in wireless communication systems. Each of the different communication systems has advantages and disadvantages related to particular system aspects.
FIG. 1 is a frequency-time representation of signals in a typical OFDM system. The OFDM system has an allocated frequency spectrum 120. The allocated frequency spectrum 120 is divided into multiple carriers, for example 130a–130d and 132a–132d. The multiple carriers in an OFDM system may also be referred to as sub-carriers. Each of the sub-carriers, for example 130a, is modulated with a low rate data stream. Additionally, as the system name implies, each of the sub-carriers, for example 130a, is orthogonal to all of the other sub-carriers, for example 130b–130d and 132a–132d. 
The sub-carriers, for example 130a–130d, can be constructed to be orthogonal to one another by gating the sub-carrier on and off. A sub-carrier, for example 130a, gated on and off using a rectangular window produces a frequency spectrum having a (sin (x))/x shape. The rectangular gating period and the frequency spacing of the sub-carriers, for example 130a and 130b, can be chosen such that the spectrum of the modulated first sub-carrier 130a is nulled at the center frequencies of the other sub-carriers, for example 130b–130d. 
The OFDM system can be configured to support multiple users by allocating a portion of the sub-carriers to each user. For example, a first user may be allocated a first set of sub-carriers 130a–130d and a second user may be allocated a second set of sub-carriers 132a–132d. The number of sub-carriers allocated to users need not be the same and the sub-carriers do not need to be in a contiguous band.
Thus, in the time domain, a number of OFDM symbols 110a–110n are transmitted, resulting in a frequency spectrum of orthogonal sub-carriers 130a–130d and 132a–132d. Each of the sub-carriers, for example 130a, is independently modulated. One or more sub-carriers 130a–130d may be allocated to an individual communication link. Additionally, the number of sub-carriers assigned to a particular user may change over time.
Thus, OFDM is a promising multiplexing technique for high data rate transmission over wireless channels that can be implemented in wireless communication systems, such as cellular communication systems supporting large numbers of users. However, cellular systems use a frequency reuse concept to enhance the efficiency of spectral utilization. Frequency reuse introduces co-channel interference (CCI), which is a major source of performance degradation in such systems. As discussed above, all users within the same cell or sector of an OFDM system are orthogonal to each other because all of the sub-carriers are orthogonal. Thus, within the same cell or sector, the multiple sub-carriers cause substantially no interference to each other. However, adjacent cells or sectors may use the same frequency space because of frequency reuse. Hence, in an OFDM system, users in different cells or sectors are sources of interference and produce the main source of CCI for adjacent cells or sectors.
The receiver may operate in a closed power control loop with a transmitter. The transmitter typically adjusts the power level transmitted on each sub-carrier in order to maintain the signal to interference plus noise ratio (SNIR) required for a certain Quality of Service (QOS) or performance level.
It is desirable to maintain the transmit power in each sub-carrier within a predefined window in order to minimize the dynamic range required of a receiver. Additionally, it is desirable to minimize the amount of CCI by minimizing the amount of interference generated by a particular transmitter.